1. Field of the Invention
The invention relates to a process for the purification of .epsilon.-caprolactam which involves hydrogenation of a water-.epsilon.-caprolactam mixture with hydrogen in the presence of a heterogeneous hydrogenation catalyst.
2. Description of Related Art
To obtain .epsilon.-caprolactam of the purity required for polymerization to nylon 6, impure .epsilon.-caprolactam, prepared by, for example, Beckmann rearrangement of cyclohexanone oxime, is subjected to a number of purification steps. One of these purification steps is the hydrogenation, as referred to here, of a mixture consisting substantially of water and .epsilon.-caprolactam and some unsaturated impurities. Hydrogenation of water-.epsilon.-caprolactam mixtures are carried out to hydrogenate these unsaturated compounds present in the impure .epsilon.-caprolactam. The presence of these unsaturated compounds are disadvantageous because they can impair the physical-mechanical properties of the nylon-6 made by polymerizing .epsilon.-caprolactam. The saturated compounds formed by hydrogenation do not adversely influence these physical-mechanical properties of the nylon-6, and moreover, these compounds are more easily removed in for example a distillation step following the hydrogenation step.
Such a process is described in EP-A-11455, or in U.S. Pat. No. 5,032,684. The process involves the purification, in a three-phase system (gas, liquid, solid), of a 75-95 wt. % aqueous .epsilon.-caprolactam mixture in which the water-.epsilon.-caprolactam mixture and gaseous hydrogen are passed from the bottom upwards through a fixed bed consisting of a supported palladium or nickel catalyst.
In this known method, the amount of hydrogen fed to the reactor is relatively large, compared with the amount of hydrogen consumed during the reaction. The residual amount of hydrogen is incinerated or recirculated to the hydrogenation reaction. In the latter case, the hydrogen is passed through a compressor to ensure that it has the correct pressure when it is returned to the reactor. A drawback of this process is that large amounts of hydrogen have to be processed which will result in more safety measures, because of the nature of hydrogen. A further drawback of the method is that the system requires a relatively large compressor to keep a large amount of hydrogen in circulation. When the residual amount of hydrogen is incinerated, economics prove to be unattractive because the burning value of hydrogen is less than the hydrogen cost price.